This invention relates to Sn-containing In oxides (hereunder sometimes referred to as ITO), a process for producing them, a coating solution containing them, and electrically conductive coatings formed of them.
Indium (In) oxides containing comparatively small amounts of Sn are commonly called ITO and are not only transparent to visible light but also high in electrical conductivity; hence, they are used to form transparent, electrically conductive films in various kinds of display device and solar cells.
Two major categories of known techniques for producing transparent conductive films from ITO are the physical approach such as sputtering and the coating process involving the application of liquid particle dispersions or organic compounds. The films formed by the coating process are somewhat low in electrical conductivity compared to the films formed by the physical approach such as sputtering; on the other hand, films of complex shape can be deposited on large-area substrates without using expensive equipment such as vacuum systems. Compared to the pyrolysis of organic compounds, the application of liquid particle dispersions permits film formation by a colder process and the resulting film having reasonable conductivity is extensively used as a shield of electromagnetic waves on CRTs. The application of the film as transparent electrodes on display devices such as LCDs and ELs is under current review. However, the coatings formed by the application of liquid particle dispersions do not have as high conductivity as the sputtered films and are still unsuitable for use on large-area CRTs or incapable of forming high-definition electrodes on display devices. It is therefore desired to develop ITO particles that can produce transparent coatings of improved conductivity which meet those use requirements.
In conductive coatings, current-carrying paths are formed by the contact between ITO particles. Since flakes, needles and tabular grains have increased chance for the creation of such current-carrying paths (ITO particles contact each other on an increased number of faces), they are used to provide increased conductivity (i.e., lowered resistivity). Known attempts to produce various particle shapes include:
(A) producing acicular ITO particles having a major axis of 5 μm or more and a major-to-minor axis ratio of at least 5 (JP 7-232920 A and JP 7-235214 A);
(B) applying the coating of conductive fine particles to titanium oxide ribbons measuring 1-100 μm long by 0.2-20 μm wide by 0.01-2 μm thick (JP 8-217446 A); and
(C) producing ITO needles having an aspect ratio of at least 4 with a major axis of 0.2-0.95 μm and a minor axis of 0.02-0.1 μm (JP 6-80422 A).
The ITO particles produced by method (A) are large and have lowered resistance values but, on the other hand, their optical characteristics such as transmittance are poor and, in particular, the scattering of light increases to produce more haze on the coatings. Method (B) uses TiO2 ribbons and is believed to produce a sufficient number of points of contact; on the other hand, the particles produced by method (B) are so large that they suffer the same problem as the particles produced by method (A). In addition, since the less conductive TiO2 particles are coated with the conductive material, the bulk resistance of the grains is high enough to prevent the development of satisfactory conductivity in the coatings. The particles produced by method (C) are smaller than those produced by method (A) and their optical characteristics are somewhat improved; on the other hand, the particles of method (C) are larger than one half the wavelength of visible light (400-700 nm) and when they are packed in the coatings, scattering of light occurs, making it impossible to assure sufficient transmittance of visible light and prevention of haze.
Haze is a characteristic property that describes the percent scattering of light and as a characteristic of transparent electrode films, smaller haze is desirable. The present inventors have shown that haze is strongly influenced by the shape of ITO particles and the degree of their dispersion.